1. Field of the Invention
The present invention relates to a system for driving a piezoelectric actuator which is applied to a camera module of a mobile phone or the like for focusing and/or zooming therein, and more particularly to a piezoelectric actuator drive system which is capable of driving a piezoelectric actuator for zooming and/or focusing in a camera module forward or backward using a phase shift of two drive waves forming a drive signal, and selectively driving two such piezoelectric actuators, thereby reducing power consumption and improving efficiency.
2. Description of the Related Art
Generally, piezoelectric actuators are transducers that convert electrical energy into a mechanical displacement or stress using a piezoelectric effect. These actuators may be, for example, a motor for zooming and focusing in a camera module of a mobile phone or the like, a cylinder, etc.
The piezoelectric actuators are driven in various manners according to the purposes and methods of use of systems to which they are applied.
Recently, such a piezoelectric actuator has been applied to a small-size camera module of a mobile phone or the like for zooming and focusing therein, resulting in a need for a drive system to drive the actuator.
FIG. 1 is a circuit diagram of a conventional piezoelectric actuator drive circuit, which is denoted by the reference numeral 2.
In the conventional piezoelectric actuator drive circuit 2 shown in FIG. 1, the reference numeral 20 denotes an initial storage unit which stores energy to drive a piezoelectric actuator 3. The initial storage unit 20 is implemented with an inductor. The reference numeral 4 denotes a voltage detector which detects a voltage across the piezoelectric actuator 3 at a predetermined time after transfer of the energy stored in the initial storage unit 20 to the actuator 3. The reference numeral 5 denotes a storage controller which compares the voltage detected by the voltage detector 4 with a voltage desired for the piezoelectric actuator 3 and, on the basis of a result of the comparison, controls the amount of energy to be stored in the initial storage unit 20 in the next driving period such that the voltage detected by the voltage detector 4 is equal to the desired voltage in the next driving period.
In FIG. 1, when a switch SW1 is turned on, energy is stored in a primary coil L1 of a transformer in the initial storage unit 20. If the switch SW1 is turned off after the lapse of a certain time, the energy stored in the initial storage unit 20 drives the piezoelectric actuator 3, which is a capacitive element, through a secondary coil L2 of the transformer and a diode D2, thereby causing a voltage Vp to be generated across the actuator 3 and then maintained at a constant level after the lapse of a predetermined time.
When a switch SW2 is turned on after the lapse of the predetermined time, the energy stored in the piezoelectric actuator 3 is discharged, so that the voltage Vp drops to 0. While the voltage Vp across the piezoelectric actuator 3 is maintained at the constant level, the voltage detector 4 detects the voltage Vp and the storage controller 5 compares the detected voltage Vp with a voltage Va desired for the piezoelectric actuator 3. The storage controller 5 then determines the amount of energy for the next driving period according to a result of the comparison and then whether to make the ON time of the switch SW1 in the circuit of FIG. 1 longer or shorter than that in the previous driving period. As a result, the desired voltage Va is rapidly applied to the piezoelectric actuator 3 by repeating the driving operation a minimum number of times.
In other words, by detecting a voltage across the piezoelectric actuator 3 which is driven at intervals of the predetermined time and determining the amount of energy to be stored in the next driving period, it is possible to maintain the voltage across the actuator 3 at a constant level and cope with capacitive variation of the actuator 3.
Details of such a piezoelectric actuator drive circuit are disclosed in Japanese Patent Laid-open Publication No. Heisei 06-62585.
However, the aforementioned conventional piezoelectric actuator drive circuit is disadvantageous in that it cannot be applied for forward or backward driving of the piezoelectric actuator and cannot selectively drive two such piezoelectric actuators.